1. Field of the Invention
The present invention relates to a digital microwave system and, more specifically, to a circuit for synchronizing transmission local oscillating frequencies in a digital microwave system.
2. Description of the Related Art
In a recent digital microwave system, co-channels are mostly used to enhance the efficiency, where the frequency of one channel is separated into a vertical polarization wave (hereinafter, referred to as "V) mode and a horizontal polarization wave (hereinafter, referred to as "H") mode and each data is loaded to the V and H modes so as to perform each function.
Upon transmission in a point-to-point microwave system operating with co-channels, a baseband signal is modulated into an intermediate frequency signal by the modulation method adapted in the above system. After that, a frequency converter uses a phase locked dielectric resonator oscillator (hereinafter, referred to as "PLDRO") having good frequency stability and phase noise characteristic at each V and H mode as a local oscillator, increases the frequency of the intermediate frequency signal and outputs it as a radio frequency signal. The V and H signals of the radio frequency outputted as described above are united into one in an OMT (orthomode transducer) after power amplification.
Meantime, a circuit for synchronizing the oscillating frequency of the transmission local oscillator typically includes a reference oscillator, a phase comparator and a loop filter, corresponding to V and H PLDROs. But, the above circuit therefor has its sophisticated construction as well as has the possibility of generating a malfunction in the system of one reference signal oscillator of the V the or H oscillators being out of order.
A technique for solving the above problem has already been proposed in Korean Patent application No. 95-26430, entitled "Circuit for Synchronizing Local Oscillating Frequency of Transmitter in Co-Channel Digital Microwave System" and assigned to the same assignee Samsung Electronics, Ltd. as in the present application, on Aug. 24, 1995. The synchronizing circuit as stated hereinbefore, is composed of two modules for co-channel operation and uses a method for controlling bias switches respectively located in the two modules by detecting oscillating signals of reference oscillators included in the modules.
However, it is troublesome for a user to intentionally operate the bias switches for operating the synchronization circuit proposed in the aforesaid patent application with a single channel. Also, each module of the synchronization circuit must have reference signal oscillation controlling ports, and bias switch controlling ports, and these ports are connected to each other through RF cables, thereby resulting in a disadvantage in operating the system. Furthermore, it is impossible to check whether or not the RF cables are properly connected.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited circuit for synchronizing transmission local oscillating frequencies and a digital microwave system as in the present invention: U.S. Pat. No. 5,592,126 to Boudewijns et al., entitled Multiphase Output Oscillator, U.S. Pat. No. 4,598,257 to Southard, entitled Clock Pulse Signal Generator System, U.S. Pat. No. 4,779,008 to Kessels, entitled Multiple Redundant Clock System Comprising A Number Of Mutually Synchronizing Clocks, And Clock Circuit For Use In Such A Clock System, U.S. Pat. No. 5,233,315 to Verhoeven, entitled Coupled Regenerative Oscillator Circuit, U.S. Pat. No. 5,301,171 to Blow et al., entitled Cross-Monitored Pair Of Clocks For Processor Fail-Safe Operation, U.S. Pat. No. 4,025,874 to Abbey, entitled Master/Slave Clock Arrangement For Providing Reliable Clock Signal, U.S. Pat. No. 5,059,926 to Karczewski, entitled Frequency Synchronization Apparatus, U.S. Pat. No. 5,422,604 to Jokura, entitled Local oscillation Frequency synthesizer For Vibration Suppression In The Vicinity Of A Frequency Converging Value, U.S. Pat. No. 5,675,620 to Chen, entitled High-Frequency Phase Locked Loop Circuit, U.S. Pat. No. 5,610,558 to Mittel et al., entitled Controlled Tracking Of Oscillators In A Circuit With Multiple Frequency Sensitive Elements, U.S. Pat. No. 5,657,359 to Sakae et al, entitled Phase Synchronizer And Data Reproducing Apparatus, and U.S. Pat. No. 5,359,298 to Abe, entitled VCO Having Voltage-To-Current Converter And PLL Using Same.